Environmental sensors in the form of optical fibers having a hollow core are known in the prior art. The hollow core of the fibers used for such sensors typically conducts light by way of a photonic band gap structure (PBG) surrounding the hollow core having a “forbidden frequency range” which corresponds to the wavelength of the light transmitted through the fiber, although hollow core fibers that conduct light via total internal reflection (TIR) for a specific range of wavelengths are also known. Such sensors may be used to sense the presence of a particular gas or liquid in the ambient environment, for example a threshold amount of carbon dioxide in the ambient air which may be indicative of a fire or other unsafe condition. In one prior art design, the hollow core of the optical fiber is exposed to the ambient atmosphere at one or both of the ends of the fiber to allow gases from the ambient atmosphere to continuously flow into a hollow core of the fiber. In operation, laser light having a wavelength which would be absorbed by the particular gas composition to be a detected is continuously conducted through the hollow core of the fiber. When such a gas is introduced into the open end of the fiber from the ambient atmosphere, it begins to flow through the hollow core, and the amplitude of the laser light transmitted through the core diminishes due to absorption of the light by the gas. In the case light transmitted through the core diminishes due to absorption of the light by the gas. In the case of the carbon dioxide example referred to earlier, the diminishment of the amplitude of the light below a certain threshold may be used to generate a signal that triggers a fire alarm circuit.
Such environmental sensors may be used to detect a broad variety of different gas compositions in the atmosphere, organic and inorganic particulates or vapor droplets, and even different liquid compositions when the fiber is immersed in a liquid. Hence such sensors have a broad applicability as detectors of not only combustion products or polluting or potentially toxic substances, but also as control or monitoring sensors in industrial manufacturing processes where the control of the composition of a particular gas or liquid is required.
Unfortunately, there are a number of shortcomings associated with such prior art optical fiber environmental sensors. As previously pointed out, access to the ambient environment is provided only at one or both of the ends of the fiber, where the relatively tiny diameter of the hollow portion is exposed to the outside atmosphere. Such restricted access to the hollow core of the sensor fiber results in a relatively long response time for such a sensor to detect a particular “target” gas or liquid. For example, for a known optical fiber sensor having a length of 21 cm, a response time of 2 minutes is required from the time that the target gas or liquid is first introduced into the hollow core of the fiber before the fiber sensor generates a signal indicating that the target gas or liquid is present. Such a long response time substantially limits the usefulness of such sensors in many applications, such as chemical manufacturing applications, where a 2 minute delay may result in the irretrievable ruin of a production run of an expensive composition.
Thus far, no satisfactory way to shorten the response time for such sensors has been found. Of course, the length of the optical fiber sensor could be shortened, but such shortening not only reduces the sensitivity of the sensor (as sensitivity is proportional to the total volume of the hollow core) but also makes it apt to generate false positives (as a single tendril of cigarette smoke curling around a 1 cm smoke detector may trigger it).
Another solution to shorten the response time might be to make the diameter of the fiber air core larger. Such a solution might be implemented by using capillary tubes with hollow cores having a diameter on the order of 1.0 mm that conduct light via grazing incidence scattering rather than by the use of TIR or a PBG. However, such capillary tube optical waveguides have high light losses of over 1 dB/m, which imposes practical limits on the length of such a sensor, and are also relatively stiff and inflexible, which prevents them from being installed in space-limited situations where a sharp bending or tight coiling of the sensor is desired. To reduce the losses associated with such a capillary tube design, the hollow interior of the tube might be coated with alternating layers of materials having sharply different indexes of refraction, thereby creating a Bragg reflector, or a single layer of a material having an index of refraction less than air. However, such coated capillary tubes would be substantially more expensive to manufacture than drawn optical fibers. Additionally, the losses would still be greater than 0.5 dB/m, and the problems associated with stiffness and inflexibility would remain. In addition, many optical sensing operations rely on nonlinear optical effects (Raman spectroscopy, for example) for which the sensitivity is proportional to the intensity (power per area) of the optical signal. A larger optical core will cause the intensity of the light in the core to decrease by a factor proportional to the square of the diameter of the core thereby reducing the device sensitivity by the same factor.
Finally, it has been proposed to laser drill a plurality of circular side holes in the fiber to better expose the hollow core to the ambient atmosphere. While such a solution may shorten the response time of the fiber sensor, the resulting response time would still be unacceptably long due the fact that access to the hollow core is still quite limited. Additionally, there is a concern in the prior art that such radially-oriented side openings create “light leaks” that limit the number of side openings that can be fabricated in such a fiber before the resulting losses become unacceptably high.
Clearly, what is needed is an optical waveguide environmental sensor that maintains the low losses, flexibility and ease of manufacture associated with optical fibers, but which substantially reduces the response time associated with fiber-based environmental sensors that rely upon a relatively small number of end or side holes to expose the hollow core of the fiber to the ambient environment.